1. Field of the Invention
The present invention relates to a method of producing a spacer arranged between a pair of substrates, and a method of manufacturing an image forming apparatus using the spacer.
2. Description of the Related Art
Known electron emitting devices include the two types of devices including a hot-cathode device and a cold-cathode device. Of these devices, known examples of the cold-cathode device include a surface conduction-type emission device, a field emission device (referred to as a FE typexe2x80x9d hereinafter) and a metal/insulating layer/metal type emission device (referred to as a MIM typexe2x80x9d hereinafter).
Examples of the surface conduction-type emission device include the device disclosed in M. I. Elinson, Radio Eng. Electron Phys., 10, 1290 (1965), and the other devices described below.
The surface conduction-type emission device utilizes the phenomenon that electrons are emitted by passing an electric current through a small-area thin film formed on a substrate in parallel to the film plane. As the surface conduction-type emission device, there have been reported the above-described device disclosed by Elinson using a SnO2 thin film, a device comprising an Au thin film [G. Dittmer, xe2x80x9cThin Solid Filmsxe2x80x9d, 9, 317 (1972)], a device comprising a In2O3/SnO2 thin film [M. Hartwell and C. G. Fonstad, xe2x80x9cIEEE Trans. EDConf.xe2x80x9d, 519 (1975)], a device comprising a carbon thin film [Hisashi Araki et al., Vacuum, Vo. 26, No. 1, 22 (1983)], etc.
A typical example of the construction of the surface conduction-type emission device is the device disclosed in M. Hartwell, et al., shown in FIG. 33. In FIG. 33, reference numeral 301 denotes a substrate, and reference numeral 304 denotes a conductive thin film of a metal oxide formed by sputtering. The conductive thin film 304 has an H-shaped planar form. The conductive thin film 304 is subjected to the electric forming described below to form an electron emitting portion 305. In this drawing, the distance L is set to 0.5 to 1 mm, and the width W is set to 0.1 mm. Although, in FIG. 33, the electron emitting portion 305 is shown in a rectangular shape at the center of the conductive thin film 304 for convenience""s sake, FIG. 33 schematically shows the electron emitting portion 305, but it does not faithfully express the position and the shape of the electron emitting portion 305.
In the surface conduction-type emission devices such as the device of M. Hartwell et al., the conductive thin film 304 is generally subjected to electric forming to form the electron emitting portion 305 before electron emission. Namely, the electric forming means that an electric current is supplied to the conductive thin film 304 by applying a constant DC voltage or a DC voltage slowly increasing, for example, at a rate of about 1 V/min., across both ends of the conductive thin film 304 to locally break, deform or deteriorate, forming the electron emitting portion 305 in an electrically high-resistance state. In the electric forming, a crack occurs in a portion of the locally broken, deformed or deteriorated conductive thin film 304. When an appropriate voltage is applied to the conductive thin film 304 after the electric forming, electrons are emitted from a portion near the crack.
Known examples of the FE type devices include the devices disclosed in W. P. Dyke and W. W. Dolan, xe2x80x9cField Emissionxe2x80x9d, Advance in Electron Physics, 8. 89 (1956), C. A. Spindt, xe2x80x9cPhysical Properties of thin-film field emission cathodes with molybdenum conexe2x80x9d, J. Appl. Phys., 47, 5248 (1976), etc.
A typical example of the construction of the FE type device is the device of C. A. Spindt et al., shown in FIG. 34. In FIG. 34, reference numeral 310 denotes a substrate; reference numeral 311, an emitter wiring comprising a conductive material; reference numeral 312, an emitter cone; reference numeral 314, a gate electrode. In this FE type device, an appropriate voltage is applied between the emitter cone 312 and the gate electrode 314 to emit electrons from the tip of the emitter cone 312.
Another example of the construction of the FE type device does not have such a laminated structure as shown in FIG. 34, but comprises an emitter and a gate electrode which are provided on a substrate in substantially parallel with the plane of the substrate,.
A known example of the MIM type device is the device disclosed in C. A. Mead, xe2x80x9cOperation of tunnel-emission devicesxe2x80x9d, J. Appl. Phys., 32, 646 (1961), etc.
FIG. 35 shows a typical example of the construction of the MIM type device. In FIG. 35, reference numeral 320 denotes a substrate; reference numeral 321, a lower electrode made of a metal; reference numeral 322, an insulating thin layer having a thickness of about 100 angstroms; reference numeral 323, an upper electrode made of a metal and having a thickness of about 80 to 300 angstroms. In the MIM type device, an appropriate voltage is applied between the upper and lower electrodes 323 and 321 to emit electrons from the surface of the upper electrode 323.
The above-described cold-cathode device can emit electrons at a relatively low temperature and thus does not require a heater, as compared with the hot-cathode device. Therefore, the cold-cathode device has a simpler structure than the hot-cathode device, thereby permitting the formation of a fine device. Even when many cold-cathode devices are arranged on a substrate with a high density, the problem of heat-melting the substrate less occurs. Also, the cold-cathode device has the advantage of a high response speed, while the hot-cathode device has a low response speed because it is operated by heating with a heater.
Therefore, applications of the cold-cathode device have been actively studied. For example, of the cold-cathode devices, the surface conduction-type emission device has the simplest structure, and thus it can easily be manufactured. Therefore, the surface conduction-type emission device has the advantage that many devices can be formed over a large area. For example, as disclosed by the applicant of this application in Japanese Patent Laid-Open No. 64-31332, a method for driving an arrangement of many devices is studied.
With respect to applications of the surface conduction-type emission device, for example, a so-called image forming apparatus such as an image display device, an image recording device, and the like, a charge beam source, etc. have been studied. Particularly, as an application to the image display device, an image display device is studied, which comprises a combination of a surface conduction-type emission device and a fluorescent material, which emits light due to electron collision, as disclosed in, for example, U.S. Pat. No. 5,066,883 and Japanese Patent Laid-Open Nos. 2-257551 and 4-28137 which relate to the applicant of this application. In the image display device comprising a combination of the surface conduction-type emission device and the fluorescent material, excellent characteristics are expected, as compared with conventional other-system image display devices. For example, the image display device comprising the surface conduction-type emission device and the fluorescent material is excellent in that a back light is not required because it is a self-emission type, and the angle of view is wider than liquid crystal display devices which have recently been popularized.
A method of driving an arrangement of many FE-type devices is disclosed in, for example, U.S. Pat. No. 4,904,895 relating to the applicant of this application. A known example of applications of the FE type device to image display devices is the flat-panel image display device reported by R. Mayer [R. Meyer, xe2x80x9cRecent Development on Microtips Display at LET1xe2x80x9d, Tech. Digest of 4th Int. Vacuum Microelectronics Conf., Nagahama, pp. 6-9 (1991)].
An example of application of the MIM type device to an image display device comprising an arrangement of many MIM devices is disclosed in Japanese Patent Laid-Open No. 3-55738 related to the applicant of this application.
Of the image display devices using the above-described electron emitting devices, a thin flat-panel image display device is a space-saving type and lightweight, and is thus attracted as a substitute for the cathode-ray tube image display device.
Thus, a flat-panel image display device is proposed, in which an electron source comprising the above electron emitting devices arranged in a matrix is contained in an airtight container. The airtight container is formed by sealing the peripheries of a faceplate on which a fluorescent material is arranged, and a rear plate on which the electron source is arranged, both of which are opposite to each other.
The inside of the airtight container is maintained in a vacuum of about 10xe2x88x924 Pa, and the image display device requires means for preventing deformation or breakage of the rear plate or the faceplate due to a difference between the pressure in the airtight container and the atmospheric pressure as the display area increases. Therefore, a structural support (referred to as a xe2x80x9cspacer or ribxe2x80x9d) comprising a relatively thin glass plate and withstanding the atmospheric pressure is provided between the rear plate and the faceplate.
A method of producing a spacer between a pair of substrates which constitute an image forming apparatus is disclosed in, for example, U.S. Pat. Nos. 4,923,421, 5,063,327, 5,205,770, 5,232,549, 5,486,126, 5,509,840, and 5,721,050, EP-A-0725416, EP-A-0725417, EP-A-0725418, EP-A-0725419, etc.
However, the image forming apparatus and flat-panel display using the above spacer has the following problems.
First, the electrons emitted from the electron emitting device near the spacer adhere to the spacer, or ions ionized by the action of the emitted electrons adhere to the spacer to possibly charge the spacer. Therefore, the orbits of the electrons emitted from the electron emitting device are bent due to charge of the spacer, and thus the electrons reach positions deviated from normal positions on the fluorescent material provided on the faceplate, thereby displaying a distorted image near the spacer.
Secondary, a high voltage Va of several hundreds V or more (for example, a high electric field of 1 kV/mm or more) is applied between the rear plate and the faceplate to accelerate the electrons emitted from the electron emitting device, thereby causing a fear of creeping discharge on the surface of the spacer. Particularly, when the spacer is charged as described above, there is the probability that discharge is induced.
In order to solve these problems, it is proposed that a small current is passed through the spacer to remove charge (Japanese Patent Laid-Open Nos. 57-118355 and 61-124031). In this case, an electrically high-resistance film (referred to as a xe2x80x9chigh-resistance filmxe2x80x9d hereinafter) is formed on a surface of a spacer base member so that a small current flows in the spacer surface. The high-resistance film used in this method comprises a metal film such as a tin oxide thin film, or tin oxide-indium oxide liquid crystal thin film, or the like.
However, in some types of images, i.e., in cases in which the driving pulse width is increased to increase the amount of the electrons emitted, the distortion of an image cannot be sufficiently decreased only by the method of removing charge by using the high-resistance film. This causes insufficient electric coupling between the high-resistance film and upper and lower substrates, i.e., between the face plate (referred to as xe2x80x9cFPxe2x80x9d hereinafter) and the rear pate (referred to as xe2x80x9cRPxe2x80x9d hereinafter), and thus a nonuniform distribution of resistance values including contact resistance occurs near the junction between both plates. As a result, the potential near the contact portions of the spacer changes to decrease the linearity of the electric field gradient of the side surface, thereby deviating the orbits of the emitted electrons away from the desired positions. This failure in potential control significantly influences the vicinity of the cathode because of its low electron kinetic energy.
In order to solve this problem, it is proposed that a low-resistance film (electrode) 25 with resistance lower than a high-resistance film 22 is provided on the end surfaces of an insulating spacer base member 21 which is in contact with a faceplate 17 and/or a rear plate 11, and the side surfaces of the space base member 21, as shown in FIG. 16. This method can ensure electric contact between the upper and lower substrates 17 and 11 and the high-resistance film 22. FIG. 16 shows an example in which the low-resistance film (electrode) 25 is arranged on the end surfaces (contact surfaces) 24 in contact with the faceplate 17 and the rear plate 11. FIG. 16 is a sectional view taken along a plane perpendicular to the plane direction of the rear plate, which includes the spacer.
On the other hand, the above two problems can be suppressed by setting Va to a low value or controlling the shape of the side surfaces of the insulating spacer base member 21 to expose the insulator to a vacuum without providing the high-resistance film 22. However, even in this case, when the potential of the end surfaces of the insulating spacer base member 21 is not defined, the orbits of the emitted electrons are changed in some cases. Therefore, even when the insulating spacer is provided between the faceplate and the rear plate, the electrode (low-resistance film) 25 must be arranged on at least one end surface of the spacer, as shown in FIG. 14. FIG. 15 is a sectional view taken along line Axe2x80x94A in FIG. 16, schematically showing the plate-shaped spacer base member 21. FIG. 8 is a perspective view showing the cylindrical spacer base member 21. In the cylindrical spacer base member 21, the diameter R of the cylinder corresponds to the length L and the thickness D of the plate-shaped spacer base member.
In addition, the term xe2x80x9cspacerxe2x80x9d is properly used with the term xe2x80x9cspacer base memberxe2x80x9d here. The xe2x80x9cspacer base memberxe2x80x9d means a member in which a film (for example, the high-resistance film 22 and the low-resistance film 25) is provided on the surfaces thereof, as shown in FIG. 16. The xe2x80x9cspacerxe2x80x9d is a general term for members provided for supporting the space between the faceplate and the rear plate, and comprises at least the spacer base member and the film (the low-resistance film (electrode)).
Japanese Patent Laid-Open No. 8-180821 and U.S. Pat. Nos. 5,561,343, 5,614,781, 5,675,212, 5,746,635, 5,742,117, and 5,777,421, and W094/18694A, W096/30926A, W098/02899, W098/03986A, and W098/28774A disclose that a metal or a high-conductivity material is formed on the end surfaces of a spacer.
The above publications disclose a method of forming a low-resistance film of a metal or a high-conductivity material on an end surface of a spacer by any one of various methods such as sputtering deposition, resistance-heating evaporation, coating, dipping, printing, and the like. Of these forming methods, the coating, dipping and printing methods (liquid phase forming method) comprising coating a liquid to the spacer base member and then baking the coating are preferred because the low-resistance film (electrode) 25 can easily be formed at low cost.
However, the use of the liquid phase forming method for forming the low-resistance film (electrode) 25 on the spacer base member 21 causes the following problem.
In the use of the liquid phase forming method, the deposition conditions for the low-resistance film (electrode) 25 greatly depend upon the surface shape of the space base member 21.
Particularly, when the spacer base member 21 has a shape in which the corners are substantially right-angled, the low-resistance film (electrode) 25 is unsatisfactorily formed at the corners in some cases. Specifically, the shape accuracy of the liquid phase film greatly depends upon the wettability of the spacer base member with the low-resistance film material, and thus the end position of the low-resistance film rises to h1 higher than the desired position h0 due to the influences of surface contamination of the spacer base member and a variation in the shape near the end surfaces thereof, as shown in FIG. 4A. As a result, the heights of the anode potential and the cathode potential are changed to lose the linearity of an electric field near the contact portions between the spacer 21 and/or the rear plate 11 and the faceplate 17, thereby deviating the electron orbit away from the desired orbit.
From the viewpoint of a decrease in formation cost, U.S. Pat. No. 5,811,927 discloses a method of bundling a plurality of spacer base members, and then forming a low-resistance film in order to permit batch processing of a plurality of spacers. When this method is applied as a pre-step of the liquid phase method, spaces partially occur between the bundled spacers, as shown in FIG. 4B. In this case, the liquid phase forming material leaking in the spaces significantly rises due to a meniscus phenomenon to extend the end of the low-resistance film to the position h2.
Furthermore, when a cylindrical spacer, a columnar spacer having a polygonal sectional shape, or a plate-like spacer having uneven side surfaces is used as the spacer base member, as shown in FIGS. 5A and B, it is very difficult to remove the spaces between the bundled spacers. Therefore, a new method is required for forming a low-resistance film on required portions of the bundled spacer base members.
An object of the present invention is to realize a method capable of precisely and efficiently forming a film on a spacer base.
Specifically, an object of the present invention is to provide a method of forming a low-resistance film (electrode) on a spacer base member without causing the above-described problems in forming the low-resistance film (electrode) at the ends of the spacer base member by using the liquid phase forming method.
In accordance with an aspect of the present invention, there is provided a method of producing a spacer provided between a first substrate and a second substrate on which an electron emitting device is arranged, the method comprising the step of forming a film on at least a portion of at least one surface of the spacer, wherein the film forming step comprises the step of preparing a bundle of a plurality of spacer base members, and the step of providing a film material on the bundle, and wherein the bundle on which the film material is provided has a mask layer for covering at least a film non-formation portion near a film formation portion of each of the plurality of spacer base members of the bundle. Here, xe2x80x9cthe film formation portionxe2x80x9d means the portion where the film material should be provided.
In this construction, the film non-formation portion is covered with the mask layer to permit the appropriate formation of a film even when a variation occurs in the conditions of providing the film material on the plurality of spacer base members which constitute the bundle. For example, in providing the film material on the bundle, there are the spacer base members in which the film material easily contacts the portions other than the film formation portion. In this case, when contact between the film non-formation portion and the film material is inhibited, contact between the film formation portion and the film material is liable to be insufficient. On the other hand, when the film material is sufficiently put into contact with the film formation portion, contact between the film non-formation portion and the film material causes a problem. The present invention is particularly effective to such a case. For example, a case in which the film material is provided on the bundle by the dipping method is considered. In this case, when the film formation portions of the respective spacer base members do not lie in the same plane, when the surface level of the film material varies, and when there is a sufficient space where the meniscus phenomenon of the film material occurs between the spacer base members, the film material is adhered to different regions of the spacer base members of the bundle in dipping of the bundle in the film material. The present invention can desirably form a film when a variation occurs in the conditions for providing the film material on the plurality of the spacer base members of the bundle. In a construction using an electron emitting device, a variation in the film formation portions of the spacers causes a variation in the electron orbits. The present invention can produce a spacer suitably used for the construction using the electron emitting device.
In the present invention, the mask layer is preferably formed on a surface of the spacer base member between the bundled spacer base members, wherein the surface of the spacer base member will be supposed to face a surface of the adjacent spacer base member each other when the plurality of the spacer base members are bundled. This is because it is difficult to control the conditions for providing the film material on a surface of the spacer base member between the bundled spacer base members, wherein the surface of the spacer base member will be supposed to face a surface of the adjacent spacer base member each other when the plurality of the spacer base members are bundled.
When the film formation portion is located in a contact surface of the spacer which is in contact with the first substrate or a contact member (wiring, an electrode, or the like) nearer to the first substrate than the spacer, or the second substrate or a contact member (wiring, an electrode, or the like) nearer to the second substrate than the spacer, electron emission is greatly influenced by the precision of film formation, particularly the degree of extension of the film formation portion to the side surfaces of the spacer. Therefore, the present invention can be particularly preferably applied to this case.
Particularly, when the film is a low-resistance film, or when the film is electrically connected to an electrode and wiring so that a potential is applied to the film, the present invention can be effectively used. Particularly, when the film has a sheet resistance value of 1xc3x97107 xcexa9/xe2x96xa1 or less, the present invention can preferably be used.
The mask layer is preferably removed when it becomes unnecessary. Specifically, the mask layer can be removed by etching. In order to preferably remove the mask layer by etching, the material and etching conditions are preferably set so that the spacer base member and the film are different in etching resistance (resistance to an etchant) from the mask layer.
Although various mask layers can be used, a mask layer which adheres to the spacer base member is preferably used for preventing adhesion of the film material to the spacer base member.
In forming the mask layer on the film non-formation portion, the mask layer is also formed on the film formation portion so that the mask can easily be formed. In this case, the mask layer formed on the film formation portion is removed before the film material is provided. This step is the step of removing the mask layer formed on the film formation portion without removing the mask layer formed on the film non-formation portion, and thus it may be called a xe2x80x9cpartially removing stepxe2x80x9d. The step of removing the mask layer before the step of providing the film material can be performed after the bundle is prepared, or performed for a plurality of spacer base members before the bundle is formed.
The step of removing the mask before the step of providing the film material can be preferably performed by physical removal. For example, the step can preferably be performed by filing or blasting.
In the present invention, a certain variation in the conditions for providing the film material on the respective spacer base members of the bundle is allowable, but the film formation portions of the respective spacer base members of the bundle are preferably located on substantially the same plane.
When the film is not completely formed only by providing the film material on the spacer base members, the step of completely forming the film is performed based on the film material. Specifically, the step of heating the film material can be preferably used. Particularly, the step of drying and/or baking the film is preferably used.
The present invention can preferably be applied to a case in which the surfaces of the spacer base members have unevenness. Also, the present invention can preferably be used for a case in which each of the spacer base members has a columnar shape.
In another aspect of the present invention, there is provided a method of manufacturing an image forming apparatus comprising a first substrate on which an image forming member for forming an image by electron irradiation is provided, a second substrate on which an electron emitting device is provided, and a spacer provided between the fist and second substrates, wherein the spacer is produced by the above-described method of producing a spacer.
For the image forming member, a fluorescent material which emits light by electron irradiation can preferably be used.
The present invention has the following effects.
The image forming apparatus using the spacer can realize high-quality display, i.e., the above-described production method can improve the shape and precision of the low-resistance film, thereby providing a spacer having suppressed beam deviation and discharge, and an image forming apparatus using the spacer.
The present invention has the second effect of obtaining high material selectivity. Namely, the production method can prevent rising of the coated liquid material, and thus makes control of wettability unnecessary or easy, thereby widening the range of selection of the base material and the liquid phase film material.
Furthermore, the geometrical requirement of the contact surfaces of the spacers, i.e., the surfaces on which the low-resistance electrode is formed, makes a patterning work substantially unnecessary. From this viewpoint, high precision and low cost can be satisfied to realize many effects, as described below. The geometrical requirement of the electrode forming surfaces means that the single spacer or a plurality of spacers arranged in the same image forming apparatus generally have a plane or common plane perpendicular to the atmospheric resistance axis.
Namely, the present invention has the third effect of permitting the step of patterning the low-resistance film formation region in a self-alignment manner by utilizing the planarity and perpendicularity of the contact surfaces. Particularly, an optical patterning method such as photolithography and an alignment work are unnecessary, and thus films (high-resistance film) on the contact surfaces of the spacers can be easily removed by a so-called physical processing method of rubbing the contact surfaces with a file in contact with the surfaces in a self-alignment manner.
Furthermore, a plurality of spacers can be bundled so that the contact surfaces lie in the same plane, thereby realizing the following effects.
The present invention has the fourth effect of permitting self-alignment coating of the low-resistance film. Namely, by forming the low-resistance film on the connected end surfaces of the spacers, the low-resistance film can be formed on many spacers at a time by using a low-cost liquid phase forming method such as dipping or the like.
The present invention has the fifth effect of widening the selectivity of the method of removing or pattering the mask layer. Namely, when the patterning step is performed for the bundled spacer base members, many formation surfaces can be formed at the same time, and not only filing but also sand blasting can be used as physical removal means because the non-formation portions of the respective spacers are masked with each other. This method can be easily applied to a step requiring a conventional pattern mask.
The present invention has the sixth effect of simplifying handing. From the viewpoint of the relation between electrical voltage resistance and the spaces between respective pixels, each of the spacers generally has a height/thickness ratio, i.e., an aspect ratio, of about 10:1 or more, and has a thin columnar structure or a thin plate structure. In some cases, in order to fix the spacers out of the image region and hold the number of the spacers assembled down, the spacers are longer than the image region to further increase the aspect ratio of the spacer base members, thereby causing the problem of chipping or breaking the spacer base members during handling in the production process. However, in the production method of the present invention, by bundling the spacers, the aspect ratio of a work can be decreased as a whole to obtain the effect of decreasing breakage of the spacers during handling.
Furthermore, in roughening the side surfaces of the spacer base members in order to suppress charging, the shape of fine irregularity is chipped during handling in some cases. However, in the production method of the present invention, by bundling the spacers, the irregular modified surfaces of the adjacent spacers are protected (shielded or masked) by each other through the mask layers during the time of handling of the bundled spacers, thereby obtaining the effect of preventing chipping of the irregular structure.
As described above, in the production method of the present invention, bundling the spacer base members can inhibit chipping of the spacer base members in handling in the production process to avoid discharge from the chipped portions, thereby providing a high-quality image display device.
The present invention has the seventh effect of improving low precision of the liquid phase film due to a meniscus. Namely, with the columnar spacers or spacers having uneven side surfaces which face each other when the spacers are bundled, it is difficult to remove the spaces between the bundled spacers. If the low-resistance film is formed on the contact surfaces of the bundled spacers with the spaces therebetween, the low-resistance film material flows into the spaces to deteriorate the formation precision of the low-resistance film, thereby causing deviation of an emitted beam. However, in the production method of the present invention, even if the spaces are present between the bundled spacers, the outer surfaces of the spacers are masked with the mask layers, and thus the low-resistance film material flowing into the spaces is finally removed together with the under mask layers in the step of removing the mask layers, thereby preventing the occurrence of the above problem.
It is important to perform the step of bundling the spacers before the step of coating the low-resistance film, but the bundling step may be performed either before or after the step of partially removing the mask layers. However, when the fifth and sixth effects are desired, the bundling step is preferably performed before the step of partially removing the mask layers.
In the above-described production method, the low-resistance film (electrode) can be uniformly and precisely formed on the end surfaces (contact surfaces) of the spacer base members at low cost by the liquid phase forming method. As a result, it is possible to obtain a high-quality image forming apparatus which has the stable orbits of the electrons emitted from the electron emitting device and no useless discharge and which can display a good image for a long time.
In the bundling step, the spacer base members are preferably bundled in either of the two directions below or a combination of the two directions.
One of the bundling directions is a xe2x80x9cparallel bundling directionxe2x80x9d in which the adjacent spacer base members have parallel lines normal to the same end surfaces. The other direction is an xe2x80x9cantiparallel bundling directionxe2x80x9d in which the adjacent spacer base members have parallel lines normal to the opposite end surfaces.
When the end surfaces of the spacer base members have a trapezoidal sectional shape, the xe2x80x9cantiparallel bundling directionxe2x80x9d is preferred.
In bundling the spacer base members, besides the method of bundling the spacers through the mask layers, the spacer base members may be bundled through an appropriate jig.
Further objects, features and advantages of the present invention will become apparent from the following description of the preferred embodiments (with reference to the attached drawings).